In Japanese Utility Model Application Laid-Open No. 1-145670, an intermediate shaft is interposed between an output shaft of a column-assist type electric power steering apparatus and a pinion shaft of a steering gear. This intermediate shaft is provided with a damper for damping a reaction force from a road surface. This intermediate shaft, however, is not extended and/or contracted in the axial direction.
FIGS. 9 to 12 show a steering apparatus for a vehicle according to a prior art. According to the prior art, an intermediate shaft which is collapsible at a secondary collision is interposed between an output shaft of a column-assist type electric power steering apparatus and a pinion shaft of a steering gear.
That is, FIG. 9 is a schematic view of the steering apparatus for a vehicle according to the prior art. In FIG. 9, a steering column 50 is rotatably supported by a steering shaft 51, and a steering wheel 52 is provided in an upper part of the steering shaft 51.
The steering column 50 is provided with a column-assist type electric power steering apparatus 53, and this electric power steering apparatus 53 is provided with an electric motor 54 for assist, a gear unit 55 serving as a reduction gear, an output shaft 56 for outputting a steering power assisted by the electric motor 54 with high torque, and so on.
The steering column 50 is also provided with a tilt mechanism 57. When an operation lever 58 is operated, the steering column 50 can be tilted around a tilt center C with respect to a bracket 59 which is fixed to the vehicle body side.
An intermediate shaft 64 is interposed between the output shaft 56 of the electric power steering apparatus 53 and a pinion shaft 61 of a steering gear unit 60 of a rack and pinion type through universal joints 62 and 63 on the upper and lower sides. This intermediate shaft 64 is composed of a female spline (or serration) shaft 65 on the upper side and a male spline (or serration) shaft 66 on the lower side which is press-fitted in this female spline shaft 65.
A male serration unit 67 is formed on the male spline shaft 66 on the front side of the vehicle, and this male serration unit 67 is fitted in a yoke 63a of the lower-side universal joint 63 and is clamped by a clamping bolt 68.
FIG. 10 is a schematic view of the steering apparatus for a vehicle according to the prior art, for showing the highest level and the lowest level of a tilt operation (a view for explaining various axial displacements).
In the steering gear unit 60, a vibration is produced from the vehicle body (on gear side) while the vehicle is running.
At assembling into the vehicle, the yoke 63a of the lower-side universal joint 63 is slid with respect to the male serration unit 67, and then the yoke 63b of the lower-side universal joint 63 is fitted to the pinion shaft 61, thereby clamping the clamping bolts 100 and 68.
On the intermediate shaft 64, the lower-side male spline shaft 66 is press-fitted in the upper-side female spline shaft 65. The intermediate shaft 64 is collapsed at a collision, but is not slid in the axial direction at a tilt operation at which the intermediate shaft 64 is normally used. Generally, the intermediate shaft 64 is collapsed when a load of not less than 500N is applied thereon in case a collision, an accident, or the like of the vehicle occurs and a displacement is generated in the axial direction.
An axial displacement due to a movement of the upper-side universal joint caused by tilt adjustment is generated between the upper-side universal joint 62 and the lower-side universal joint 63. In this case, there is provided no mechanism which slides with a low load between the upper-side universal joint 62 and the lower-side universal joint 63, so that an axial load is generated between these joints.
Further, when torque is transmitted in a state that a position at which the column-assist type electric power steering apparatus 53 is installed is deviated from the designed position, a torque fluctuation becomes a little larger with respect to the designed value.
FIG. 11 is a view for explaining that an inter-joint length changes at the time of tilt adjustment.
The relation that LA<LB<LC is established when the inter-joint length (L) is LA at the tilt highest level position, LB at a tilt neutral position, and LC at the tilt lowest level position.
FIG. 12 is a partially cut-away cross sectional view of the intermediate shaft according to the prior art. This intermediate shaft 64 is composed of a female spline (serration) shaft 65 on the upper side and a male spline (serration) shaft 66 on the lower side which is press-fitted in the female spline shaft 65.
The female spline shaft 65 works also as a yoke of the upper-side universal joint 62. A male serration unit 67 is formed on the male spline shaft 66 on the vehicle front side, and this male serration unit 67 is fitted in a yoke 63a of the lower-side universal joint 63 and is clamped by a clamping bolt 68.
The column-assist type electric power steering apparatus is provided with the following function. That is, when the driver performs steering through the steering wheel 52, a steering torque thereof is detected to be calculated by CPU, and the electric motor 54 is driven on the basis of a result of the calculation. An output shaft of the electric motor 54 and a steering shaft are engaged with the gear unit 55 of a worm and wheel type, and the steering power is assisted by the power of the electric motor 54.
The intermediate shaft attached to the column-assist type electric power steering apparatus has the following functions.                The intermediate shaft is required to have the structure which can endure high torque transmission since it transmits high torque constantly.        There should be generated no backlash in the direction of rotation since it controls the assist of the steering torque.        The intermediate shaft has a collapsing mechanism for absorbing displacement from the front part of the vehicle at a vehicle collision in order to protect the driver.        
As described above, the above functions are given to the intermediate shaft according to the prior art. However, in terms of the steering stability, the assembling performance and the cost, these functions are not always sufficient.
In order to satisfy the above items, such a function is required which can extend and contract the intermediate shaft with a low stable sliding load so that the intermediate shaft can be moved easily with a hand and can transmit high torque without backlash.
The followings are reasons for requiring this expanding/contracting function.                At steering, a low stable steering shaft rotating torque property which is not affected by a deviated position of installation in the vehicle is to be obtained.        At running of the vehicle, the intermediate shaft is to absorb a vibration and a displacement from the steering gear side so as to make it difficult to transmit unpleasant vibration or sound onto the steering wheel.        At tilt adjustment, the intermediate shaft is to easily absorb an axial displacement when the steering wheel is tilted in an up-and-down direction and the length of the intermediate shaft is changed so as to prevent an extra axial load or moment from being applied on the whole steering shaft.        At assembling, the intermediate shaft is to be extended and contracted freely so as to be easily assembled in the vehicle, whereby one clamping position in the clamping work of the clamping bolt can be omitted.        
In European Patent Application Laid-Open No. EP1078843A1, a member for preventing backlash in the direction of rotation, which is required for a steering shaft, takes a retainer-like shape of plastic. Therefore, small gaps between the male shaft, the female shaft and the needle rollers are adjusted by such plastic members.
However, plastic products are not satisfactorily abrasion-proof and it is difficult to maintain the performance of such products without backlash for a long term use. For this reason, when abrasion is produced, there arises a problem that the backlash is generated on the steering shaft. Since the structure is also required to allow relative movement in the axial direction so that the male shaft and the female shaft are moved relatively to each other, it is required to provide a gap in a sliding portion so that backlash can not be structurally prevented completely.
In FIGS. 1 to 5 of German Patent No. DE3730393C2, in order to apply preload, there is required a structure in which leaf springs provided in different grooves are connected to-each other. In other cases, there is required a structure in which elastic members are laid under leaf springs so as to generate preload in the radial direction.
In these structures, since leaf springs having different curvatures are provided in grooves of the male shaft and the female shaft having the curvatures, it is very difficult to obtain a large amount of flexure of the leaf springs. As a result, it is very difficult to provide the leaf springs with a flexural amount which can allow fluctuations in the processing precision of the male shaft and the female shaft.
When torque is inputted, the male shaft, the leaf springs, balls and the female shaft are mutually contracted to transmit the torque, so that the points contacting with the balls have very high surface pressure, and the steering shaft can highly possibly not satisfy the required torque transmitting performance and the life as that used for a vehicle (particularly when the shaft is combined with the column-assist type electric power steering which is required to transmit high torque).
Since an outer leaf spring tends to slide sideways with respect to the female shaft at the time of torque transmission and hysteresis is easily generated, a structural problem that the magnitude of this hysteresis can not be controlled.
However, in the prior art described above, even if taking Japanese Utility Model Application Laid-Open No. 1-145670, the prior art illustrated in FIGS. 9 to 12, European Patent-Application Laid-Open No. EP1078843A1 described above, and German Patent No. DE3730393C2 described above into consideration, there is provided no telescopic shaft which has such a performance as can be combined with the column-assist type electric power steering apparatus. As a result, it is difficult to obtain a steering system having such an excellent performance as to satisfy the items described above.
Under such circumstances, it is expected that a total steering system in which a telescopic intermediate shaft with a high performance to satisfy the above-described requirements is combined with the column-assist type electric power steering apparatus is to be developed.
The present invention has been contrived taking the above circumstances into consideration, and an object thereof is to provide a steering apparatus for a vehicle which can transmit high torque without backlash when being provided with a column-assist type electric power steering apparatus and which can extend and contract an intermediate shaft with such a low stable sliding load as can move the intermediate shaft easily with a hand.
In order to achieve the above object, according to the present invention, there is provided a steering apparatus for a vehicle in which a steering power of a steering shaft is assisted by a column-assist type electric power steering apparatus so that the steering power having increased torque is transmitted to steer the wheels by means of a steering mechanism in its turn, characterized in that:
a telescopic shaft with a male shaft and a female shaft fitted to each other to be mutually unrotatable and slidable is interposed between an output shaft of the column-assist type electric power steering apparatus and an input shaft of the steering mechanism.
As described above, according to the present invention, since the telescopic shaft with the male shaft and the female shaft fitted to be mutually unrotatable and slidable is interposed between the output shaft of the column-assist type electric power steering apparatus and the input shaft of the steering mechanism, it is possible to realize a stable sliding load and to prevent backlash without fail, thereby transmit torque in a state of high rigidity.
With this arrangement, the following items which are required when the intermediate shaft is combined with the column-assist type electric power steering apparatus as described above can be fully satisfied.                At steering, a low stable steering shaft rotating torque property which is not affected by a deviated position of installation in the vehicle is to be obtained.        At running of the vehicle, the intermediate shaft is to absorb a vibration and a displacement from the steering gear side so as to make it difficult to transmit unpleasant vibration or sound onto the steering wheel.        At tilt adjustment, the intermediate shaft is to easily absorb an axial displacement when the steering wheel is tilted in an up-and-down direction and the length of the intermediate shaft is changed so as to prevent an extra axial load or moment from being applied on the whole steering shaft.        At assembling, the intermediate shaft is to be extended and contracted freely so as to be easily assembled in the vehicle, whereby one clamping position in the clamping work of the clamping bolt can be omitted.        The intermediate shaft is required to have the structure which can endure high torque transmission since it transmits high torque constantly.        Since no backlash is generated in the direction of rotation, the assist of the steering torque can be controlled more correctly.        The intermediate shaft has a collapsing mechanism for absorbing a displacement from the front part of the vehicle at a vehicle collision in order to protect the driver.        The intermediate shaft is extended and contracted easily at tilt and telescopic operations so that it is possible to obtain an optimal position for the driver and to obtain a stable steering property.        